What's the difference between Chromium OS and Google Chrome OS?

Google Chrome OS is to Chromium OS what Google Chrome browser is to Chromium.

Chromium OS is the open source project, used primarily by developers, with code that is available for anyone to checkout, modify, and build.

Google Chrome OS is the Google product that OEMs ship on Chromebooks for general consumer use.

Some specific differences:

The two projects fundamentally share the same code base, but Google Chrome OS has some additional firmware features, including verified boot and easy recovery, which require corresponding hardware changes and thus also don't work in Chromium OS builds.

Google Chrome OS runs on specially optimized hardware in order to get enhanced performance and security.

Chromium OS does not auto-update (so that changes you may have made to the code are not blown away), whereas Google Chrome OS seamlessly auto-updates so that users have the latest and greatest features and fixes.

Google Chrome OS is supported by Google and its partners; Chromium OS is supported by the open source community.

Google Chrome OS includes some binary packages which are not allowed to be included in the Chromium OS project. A non-exhaustive list:

Where can I download Google Chrome OS?

Google Chrome OS is not a conventional operating system that you
can download or buy on a disc and install. As a consumer, the way you
will get Google Chrome OS is by buying a Chromebook that has Google Chrome
OS installed by the OEM. Google Chrome OS is being developed to run on
new machines that are specially optimized for increased security and
performance. We are working with manufacturers to develop reference
hardware for Google Chrome OS.

Where can I download Chromium OS?

If you are the kind of developer who likes to build an open source operating system from scratch, you can follow the developer instructions to check out Chromium OS, build it and experiment with it. A number of sites have also posted pre-built binaries of Chromium OS. However, these downloads are not verified by Google, therefore please ensure you trust the site you are downloading these from.

Keep in mind that Chromium OS is not for general consumer use.

I am a hardware manufacturer, who can I talk to about making a Chrome OS product?